The present invention relates to a waxy maize starch derived from a plant which is heterozygous for the recessive sugary-2 (su.sub.2) allele. Such starch exhibits good freeze-thaw stability, relatively high viscosity, and high pasting temperature, yet is readily recoverable with good yields by wet milling and has a starch content and structure comparable to that of waxy maize.
Starch, in general, contains two types of polymers, amylose which is essentially linear and amylopectin which is branched. Waxy maize starch contains a significantly higher level of amylopectin than common corn starch and is therefore of particular value in applications where the starch serves primarily as a thickener or stabilizer, and where a stable sol which resists retrogradation is desired.
To meet the demanding needs of the food industry, native waxy maize starch is often modified by numerous techniques known in the industry to change the behavioral characteristics yet still essentially retain the caloric value of the unmodified waxy maize starch. In particular, modifications are often made to increase process tolerance and stability in an aqueous dispersion.
One common type of modification is cross-linking. When an aqueous dispersion of native waxy maize starch is heated, the starch granules begin to swell, and the dispersion develops a short, salve-like texture which is important in imparting palatability and thickening in food systems. However, during the process of cooking native starches, this textural state, particularly with waxy starches, rapidly changes to an elastic, rubbery state in which the swollen granules rupture. Minor variations in cooking time, temperature, and concentration as well as shear and pH are sufficient to effect this transformation. Cross-linking acts to strengthen the granules by reinforcing the hydrogen bonds which are responsible for holding the granules intact and thus are used to overcome the extreme sensitivity of the swollen starch granules to handling and processing conditions.
Aqueous dispersions of cross-linked starch are often used under conditions which involve prolonged storage at relatively low temperature and/or exposure, at times, to repeated freezing and thawing cycles. For example, starch dispersions are used in numerous food products such as canned and frozen foods, particularly fruit preps, pies, soups, and the like. In the case of canned food products, these are often stored in warehouses which have no heating facilities and may therefore be at very low temperatures for prolonged periods and may freeze during shipping. Frozen foods are also subject to long term storage at very low temperatures as well as freezing and thawing during distribution. Under such circumstances involving exposure to low temperature, there is a distinct loss in the hydrating power of the starch which is present in such food products, thereby resulting in syneresis, an extrusion of liquid, together with a marked deterioration in the texture, color and clarity of the food product. While sols of waxy maize starch are superior in stability to those of regular corn starch, even they are prone to intermolecular association during storage at or near freezing temperature.
Attempts to overcome these difficulties are known in the art and include the introduction of blocking groups onto the starch molecule by means of various chemical derivatization reactions, for example, reacting the starch with a monofunctional reagent to introduce substituents such as hydroxypropyl, phosphate, acetate or succinate groups. Such substituents stabilize the starch by interfering with the association between molecules or portions of the same molecule, thus reducing the tendency of the substituted starches to lose their hydration ability, clarity, and short, smooth texture on storage particularly at low temperatures.
These derivatization reactions alone may be carried out on native starches to improve their low temperature stability, but are frequently combined with cross-linking to obtain starches for use as thickeners in canned pie filings, retorted puddings and the like which will keep food from losing its clarity and texture when exposed to low or freezing temperatures.
The use of chemically modified starches, such as cross-linked starches, in food products is regulated by the Food and Drug Administration. These regulations limit the degree of modification by defining the maximum amount of chemical reagent which can be used and the levels of by-products in the resultant starch.
In recent years there has been a trend toward developing starches which have all the properties of a modified starch, but with much lower levels of, or even no, chemical treatment. For example, attempts have been made to increase the low temperature stability of starches by using .beta.-amylase to enzymatically hydrolyze cross-linked waxy starches (Wurzburg, U.S. Pat. No. 3,525,672) and native waxy starches (EP 574 721). Such hydrolysis shortens or removes the outermost .beta.-chains of the starch molecule. Thus, the possibility of association on the part of these branches is lessened, resulting in significant reduction of syneresis and gelling of the food products during exposure to low or freezing temperatures.
The recessive sugary-2 allele of maize was identified in 1934 and its dosing effect on dent corn starch was studied by Campbell, et al. ("Dosage Effect at the Sugary-2 Locus on Maize Starch Structure and Function," Cereal Chemistry 71(5):464-468 (1994)). Campbell discovered that there was a dosing effect for gelatinization onset, gelatinization peak, range total enthalpy, retrogradation, viscosity, and gel strength; that is that there were incremental changes in these properties with the increased presence of the recessive sugary-2 allele.
Work was also done on the starch extracted from a homozygous recessive su.sub.2 waxy maize plant. It was discovered that this naturally short .beta.-branched starch has excellent freeze-thaw stability. See Wurzburg, U.S. Pat. No. 4,428,972. However, the plant has a low starch content and milling yield and the starch derived therefrom has fragile granules and a low viscosity.
Surprisingly, it has now been discovered that the waxy maize starch derived from the grain of a plant which is heterozygous for one or two doses of the recessive sugary-2 allele has excellent freeze-thaw stability and a chain-length distribution similar to waxy maize. In addition, the plant starches have a relatively high peak viscosity. Further, the plant has a relatively high starch content and good milling yield.